The present invention relates to a broadband directional coupler in microstrip technique.
Such directional couplers are used in high and ultra high frequency applications for coupling a well defined, generally small portion of a signal guided in a first line to a second line and thus to extract it for control or monitoring purposes.
Such a directional coupler generally comprises a substrate on which the two lines extend through a coupling zone in which they influence each other capacitively and magnetically, without direct coupling between the two.
On the first line, signals in general can transit through the coupler in opposite directions.
For many applications, it is important to be able to extract only signals which propagate in one of the two opposite directions in the first line or to be able to distinguish between signals propagating in opposite directions, in order to be able to distinguish, by means of a directional coupler located between a transmitter power stage and an antenna, the output signal of the power stage from a signal eventually reflected by the antenna. For this purpose it is necessary that the directional coupler has a high level of directivity, i.e., if an input signal transits the directional coupler in one direction on the first line, the signal thus induced in the second line shall predominantly propagate in one direction only.
The directivity is achieved by a combined use of capacitive and magnetic coupling. If a point on the second line is capacitively influenced by a signal guided in the first line, signals with equal phase will propagate from it in both directions of the second line. In case of magnetic coupling of a point, the signals propagating from it in opposite directions differ in phase by 180°. This property is made use of in directional couplers by combining capacitive and magnetic coupling such that both contribute to the same extent to the signal generated in the second line, whereby the contributions to a signal propagating in a first direction in the second line interfere constructively and those for a signal propagating in an opposite directions interfere destructively.
Such an effect cannot be achieved by simply arranging the first and second lines in parallel in the coupling zone, for in such a case the coupling is quite predominantly of magnetic type.
It is therefore necessary to find a geometry for the various lines of a directional coupler that favor capacitive coupling over magnetic coupling. A known solution of this problem is shown in FIG. 1. Between input/output ports 1, 2, 3, 4 of the directional coupler, the two lines comprise two coupling lines 5, 6 that extend in parallel to each other in a predetermined distance and influence each other mainly magnetically to an extent depending on their distance. At each end of the parallel coupling lines 5, 6, there are regions with strong capacitive coupling formed by conductor portions 7 extending towards the other coupling line and providing locally a predominantly capacitive coupling.
A similar design is known from U.S. Pat. No. 5,767,763 A1. Here the coupling lines are formed of two portions perpendicular to each other, the ends of which are facing each other and form the regions of strong capacitive coupling.
With a coupler designed according to the prior art scheme of FIG. 1, a good directivity can be achieved for frequencies, the wavelength of which in the lines corresponds to four times the length of coupling lines 5 and 6, respectively. When this frequency is departed from, the relative phase of the capacitive contributions of the projecting conductor portions 7 changes. Due to the design principle, a satisfying directivity can thus only be obtained within a narrow band around this one frequency.
In order to achieve a more broadband directional coupler, it would be desirable to reduce the length of the coupling zone. However, this is difficult with the prior art design principle, because if a coupling capacity is formed between the first and second lines, this always implies the occurrence of a parasitic capacity between the lines and a ground plane which is located on a side of the substrate opposite to the lines. The existence of this parasitic capacity disturbs the behavior of the coupling zone. Conventionally, such disturbances are compensated by providing the coupling capacities in pairs at a distance of λ/4, λ being the wavelength corresponding to the center frequency of the frequency band in which the coupler is effective. This distance λ/4 therefore defines a minimum size which the coupling zone must have. If the coupling zone were to be made smaller than this size, the disturbances due to the existence of the coupling capacity would have to be compensated by means of inductive or capacitive auxiliary structures located outside the coupling zone. Since these again must have a wavelength dependent distance from the coupling zone, the compensation can only be effective for a limited frequency band. Therefore, the bandwidth in which a directional coupler has a satisfying directivity can only be improved within narrow limits with the prior art design principle, and a miniaturization of the directional coupler is hardly possible.
Another disadvantage of the prior art design principle is that the coupling lines 5, 6 form a system capable of resonance at the operating frequency of the directional coupler. The resonant enhancement of the currents on the coupling lines leads to increased eradiation compared with non-resonant line portions and thus to losses on the one hand and to a strong influence on the currents in the directional coupler by fields that are reflected at the metallization of the opposite substrate side and reach the coupling zone with a phase delay. Since at present, techniques for preventing or reducing the eradiation are lacking, it is attempted to minimize their disturbing influence by using substrates that are as thin as possible and only induce a moderate phase delay between the currents in the coupling zone and the fields reflected back into it. The mechanical sensitivity of these thin substrates affects the durability of couplers manufactured on them and their production yield.